1. Field of the Invention
The present invention relates to an apparatus for detecting foreign matter such as small dust particles present on a test specimen by means of the scattering phenomenon of a laser beam or the like, and more particularly to an apparatus for detecting foreign matter present on a photomask or a reticle for the manufacture of large-scale integrated circuits (LSI), said apparatus being provided with a mechanism for preventing unnecessary so-called stray light generated when the laser beam is incident thereon.
In an apparatus for detecting foreign matter present on such photomask or reticle, the detection can be achieved by scanning the specimen, for example, with a laser beam and converting the light scattered by the foreign matter into an electric signal by means of photoelectric converting means such as a photodiode or a photomultiplier.
When a specimen such as a photomask or a reticle is illuminated by a laser beam, foreign matter present on the specimen usually generates omnidirectional scattering while the edges of the circuit pattern, such as the edges of chromium masks, formed close on said specimen generates light scattering with directivity. Therefore, in order to distinguish the scattered light from such pattern edges, it is possible to provide plural photoelectric converting means in such positions as to inspect the area illuminated by the laser beam from different angles and to identify the presence of foreign matter only when all photoelectric converting means receive any scattered light.
In such inspection apparatus, the time required for inspection and the accuracy of detection depend on whether the specimen is perpendicularly or obliquely illuminated by the laser beam. For example, the inspection can be carried out within a relatively short time if the specimen is scanned by an oblique incident laser beam. However, in case a glass-based specimen, such as a photomask or a reticle, receives an oblique laser beam at a certain angle, a part of said laser beam travels inside the glass plate and again emerges therefrom as a stray light. This phenomenon will be further explained later, but the afore-mentioned photoelectric converting means may receive said stray light. Also the laser beam directed to the photomask or reticle is often reflected at the incident surface or is transmitted through a transparent area outside the circuit pattern, and such reflected or transmitted laser light is further reflected or scattered in the inspecting apparatus to generate stray light. Such stray light, being approximately equal to or often stronger, in intensity, than the scattered light from the foreign matter, reduces the ability of foreign matter detection and eventually leads to an erroneous detection.